Neural circuits underlying ketamine-induced oculomotor behavior in the rat: 2-deoxyglucose studies.

Time-related changes in oculomotor function and of metabolic activity patterns in selected brain networks, as assessed by the quantitative 2-deoxyglucose technique, were investigated in Long-Evans rats following intraperitoneal administration of a ketamine anesthetic dose. During ketamine-induced anesthesia a nystagmic-like behavior was present, characterized by uni-directional slow ocular drifts with superimposed paroxystic bursts of quick (saccadic-like) eye movements; all quick movements were executed in the horizontal direction, were strictly confined to an ocular hemifield of vision, and were followed by a backward (centripetal) drift. A metabolic hyperactivity was found in the dorso-medial shoulder region of the frontal cortex, corresponding to the rat saccadic cortical generator area, whereas functional activity levels were decreased in cerebellum and in several brainstem regions, including portions of the reticular formation and medial vestibular nuclei, putatively indicated as the locus of the oculomotor neural integrator. Starting 2 h after drug injection, a gradual recovery of oculomotor function occurred, with the disappearance of slow ocular drifts. However, an almost uninterrupted sequence of individual saccades was still present. Significant metabolic increases were found at this time in the cingulate and frontal cortex, basal ganglia, superior colliculus, paramedian reticular formation and oculomotor nuclei, the cerebellar vermis and paraflocculus. In medial vestibular nuclei, metabolic levels were undistinguishable from controls. These results suggest different concentration-dependent actions of ketamine on cortical and subcortical circuits involved in saccade generation and gaze holding. These effects are likely to be related at least in part to antagonism of N-methyl-D-aspartate receptor-mediated functions.

Intraocular injection of APB decreases the metabolic response of the rat superior colliculus.

The functional activity of the superior colliculus (SC) following flash stimulation was examined by means of 2-deoxyglucose (2DG) method in monocularly enucleated Long-Evans rats intraocularly injected with a solution of 2-amino-4-phosphonobutyric acid (APB), able to silence the ON retinal activity. After APB injection SC layers recipient of retinal axons suffered a sustained 2DG decrease as compared to controls. Moreover, whereas glucose utilization in normal rat SC appears unevenly distributed and closely related to retinal axon density, by contrast, APB-treated rats showed a rather uniform metabolic activity throughout the SC surface. Furthermore, we calculated the 2DG uptake percentage reduction in 12 SC loci of APB rats in respect to homologous loci of controls: glucose utilization decreases at the loci largely differ from each other, showing that retinal axons carrying ON activity are dispersed over the SC in a mosaic pattern. Thus, the SC metabolic map should be representative of its retinotopic organization which, in turn, depends on the asymmetric distribution in the rat's retina of W-like as well as of other ganglion cell types.

Kainic acid differently affects retinal projections to different pretectal nuclei.

Kainic acid (KA) damages retinal cells, thus impairing axonal anterograde transport of labeled aminoacids when injected intravitreally. In this study, Long-Evans rats were injected with KA into one eye, and seven days later were binocularly injected with 14C-valine. The extent of residual retinal afferents to two pretectal nuclei was calculated as the percentage of the contralateral, intact side. Projections to the nucleus of the optic tract (first relay station of the optokinetic pathway) appear significantly more affected than those to the olivary pretectal nucleus (involved in the pupillary light response). These results suggest a correlation between the functional properties of retinal ganglion cells and distinctive biochemical characteristics, such as their susceptibility to KA.

Functional impairment of the rat superior colliculus after kainic acid intraocular injection: a 2-deoxyglucose study.

Long Evans rats monocularly injected with the kainic acid (KA), were exposed to "tonic" (diffuse steady light, stationary pattern, total darkness) and "phasic" (flashing, moving pattern) stimulations. By means of the autoradiographic 2-deoxyglucose (2DG) technique we assessed the functional activity of the Superior Colliculus (SC) contralateral to the injected eye as compared to the normal eye SC. In the control SC all "tonic" stimulations determined low 2DG uptake not modified by the intraocular KA injection. On the contrary, "phasic" stimulations elicited a strong 2DG consumption in the normal SC, with a peculiar pattern of distribution depending on the kind of stimulus. Considering the total 2DG uptake as the added intrinsic and afferent metabolism, KA was able to affect only the latter, decreasing two-fold that expected for the afferent input loss. These findings can suggest a possible KA effect on off-line ganglion cells and, on the other side, they confirm the role of the SC in discriminating "phasic" and sudden phenomena from "tonic" and continuous ones.

Correlation between retinal afferent distribution, neuronal size, and functional activity in the guinea pig medial terminal accessory optic nucleus.

The intrinsic morpho-functional organization of the medial terminal nucleus of the accessory optic system was investigated in the guinea pig. The distribution of the retinal afferents, as assessed by the axoplasmic transport of 14C-valine, showed a remarkable asymmetry within the nucleus. Thus, while the retinal terminal field covered the entire medial terminal nucleus, by far the largest density of labeled retinofugal axon terminals was found within its dorsal division. In this same portion of the nucleus, we found the greatest density of large cells and the maximum intensity of functional activation, this latter as estimated by the increase in metabolic activity of neurons using the 14C-2-deoxyglucose autoradiographic method in the vertical and horizontal whole-field movement in the visual field.

The interstitial nucleus of the superior fasciculus, posterior bundle (INSFp) in the guinea pig: another nucleus of the accessory optic system processing the vertical retinal slip signal.

As in rabbit, gerbil, and rat, the guinea pig interstitial nucleus of the superior fasciculus, posterior bundle (INSFp) is a sparse assemblage of neurons scattered among the fibers forming the fasciculus bearing this name. Most of the INSFp neurons are small and are ovoid in shape. Interspersed among these, are a few larger, elongated neurons whose density becomes greater and whose shape becomes fusiform in correspondence to the zone of transition from the superior fasciculus to the ventral part of the medial terminal nucleus (MTN). Like the MTN, the INSFp is activated by retinal-slip signals evoked by whole-field visual patterns moving in the vertical direction, as shown by the increase of 14C-2-deoxyglucose (2DG) uptake into this nucleus. At the same level of luminous flux, neither pattern moving in the horizontal direction nor the same pattern held stationary can elicit increases in the INSFp 2DG assumption. The specificity of the observed increases in metabolic rates in INSFp following vertical whole-field motion suggests that this assemblage of neurons relays visual signals used in the control of vertical optokinetic nystagmus.

Tonic pain time-dependently affects beta-endorphin-like immunoreactivity in the ventral periaqueductal gray matter of the rat brain.

beta-Endorphin-like immunoreactivity (B-EP-LI) levels have been investigated in the ventral periaqueductal gray matter (vPAG) of rats killed 30, 60 or 120 min after the subcutaneous injection of dilute formalin (0.08 ml, 5%) in one fore- or hindpaw, or comparable handling. B-EP-LI was estimated by radioimmunoassay, using an anti-camel B-EP serum directed against the C-terminal portion of B-EP molecule. In both fore- and hindlimb groups vPAG B-EP-LI values were significantly increased 60 and 120 min after the injection relative to controls. Values from animals killed 120 min after formalin injection were higher than the ones at 30 and 60 min, forelimb effects being quantitatively more pronounced. The increase in B-EP-LI appeared distributed along the whole rostrocaudal extent of the region.

Ketamine effects on local cerebral blood flow and metabolism in the rat.

The effects of an anesthetic dose (100 mg/kg) of ketamine, a phencyclidine derivative, on local rates of cerebral glucose utilization (LCGU) and CBF (LCBF) have been investigated by the quantitative [14C]2-deoxy-glucose and [14C]iodoantipyrine techniques in the unparalyzed, spontaneously breathing rat. In ketamine-injected animals, LCGU was significantly increased in some limbic structures and decreased in inferior colliculus, vestibular, and cerebellar nuclei. The degree and spatial distribution of drug-induced changes was similar for local blood flow rates, LCBF being increased in limbic regions and decreased in the inferior colliculus. Although Paco2 values were higher in anesthetized animals, the pattern of LCBF/LCGU ratios was not significantly affected by ketamine in the 36 brain regions examined in this study. So, at least in the rat and at the anesthetic level studied here, a net vasodilatory in vivo effect was not observed. These results support the hypothesis that CBF changes induced by the drug in animals and man are primarily related to the metabolic effects exerted by ketamine on cerebral structures.

Vertical and horizontal visual whole-field motion differently affect the metabolic activity of the rat medial terminal nucleus.

The metabolic activity of the medial terminal nucleus (MTN) of the Accessory Optic System was studied by means of the [14C]2-deoxyglucose (2-DG) method in Long-Evans rats exposed to moving and stationary visual stimuli. In particular we explored the rate of local cerebral glucose utilization (LCGU) and the spatial distribution of 2-DG uptake within MTN related to visual stimuli capable of triggering optokinetic nystagmus. It was found that increases in MTN metabolism accompanied the retinal slip signals evoked by whole-field visual patterns moving in the vertical as well as in the horizontal direction. At the same level of luminous flux neither the same but stationary pattern, nor constant, diffuse illumination were able to elicit comparable changes in MTN metabolic rates. The effects of vertical and horizontal motions differed, however, from each other. In binocular testing LCGU rates resulted significantly higher after vertically moving patterns and upon the same stimulus condition the spatial distribution of 2-DG matched very closely the spatial distribution of the retinal afferents and the cellular density within MTN, in sharp contrast with the diffuse spreading out of the label across the nucleus following horizontal motion. In monocular testings only the vertically moving patterns were able to increase LCGU rates significantly and then in contralateral MTN alone. However, comparison between the levels of glucose consumption measured in binocular and in monocular vision also showed the involvement of the uncrossed retinal path in relaying the retinal slip signals to MTN. No difference in LCGU and in spatial distribution of the label were finally observed in relation to the upward or to the downward direction of the moving pattern.

Computer-assisted analyses of [14C]2-DG autoradiographs employing a general purpose image processing system.

A general purpose image processing system is described including B/W TV camera, high resolution image processor and display system (TESAK VDC 501), computer (DEC PDP 11/23), and monochrome and color monitors. Images may be acquired from a microscope equipped with a TV camera or using the TV in direct viewing; the A/D converter and the image processor provides fast (40 ms) and precise (512 X 512 data points) digitization of TV signal with a 256 gray levels maximum resolution. Computer programs have been developed in order to perform qualitative and quantitative analyses of autoradiographs obtained with the 2-DG method, which are written in FORTRAN and MACRO 11 Assembly Language. They include: (1) procedures designed to recognize errors in acquisition due to possible image shading and correct them via software; (2) routines suitable for qualitative analyses of the whole image or selected regions of it, providing the opportunity for pseudocolor coding, statistics, graphic overlays; (3) programs permitting the conversion of gray levels into metabolic rates of glucose utilization and the display of gray- or color-coded metabolic maps.

Optokinetic visual detection in the rat visual centres. A [14C]-2-deoxy-D-glucose study.

Following forty-five min of binocular optokinetic stimulation (OKS) the autoradiographic maps of [14C]-2-deoxy-D-glucose (2DG) assumption of Long-Evans brain reveal clearly different patterns of optical density within visual centres. The most superficial layers of superior colliculus (SC) and a pretectal area including the nucleus of the optic tract (NOT) appear symmetrically, strongly darker than other visual structures such as lateral geniculate nucleus (LGN) and visual cortex (VC). Whereas the lack of metabolic increase at LGN and VC levels entirely confirms the non-involvement of the geniculo-cortical path in mediating the optomotor response following OKS in Rodents, it is postulated that the symmetrical increase of 2DG uptake even upon unidirectional OKS found even at pretectal level may represent a commissural transfer of visual information between homologous pretectal areas like the nuclei of the optic tract.